fix: avoid channel get changed when concurrent subscribe happens (backport: #1773)

[mmsqe]

for more info: evmos#1773

Description

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #247 (86c6a7e) into release/v0.20.x-cronos (bad31ab) will decrease coverage by 4.13%.
The diff coverage is 100.00%.

@@                    Coverage Diff                     @@
##           release/v0.20.x-cronos     #247      +/-   ##
==========================================================
- Coverage                   56.68%   52.56%   -4.13%     
==========================================================
  Files                         108      113       +5     
  Lines                       10243    11073     +830     
==========================================================
+ Hits                         5806     5820      +14     
- Misses                       4190     5006     +816     
  Partials                      247      247              

Impacted Files	Coverage Δ
...c/namespaces/ethereum/eth/filters/filter_system.go	7.90% <100.00%> (ø)

... and 4 files with indirect coverage changes